1. Field of the Invention
The present invention is in the technical field of cardiac and autonomic nervous system function monitoring in human beings. More particularly, the present invention is in the technical field of Heart Rate Variability (HRV) measurements in human daily life.
2. Description of the Related Art
Heart Rate Variability (HRV) is based on the measurement of the time difference between each heartbeat, i.e., the beat-to-beat variability when processed using time domain, frequency domain or other measures, as described in “Heart rate variability: Standards of measurement, physiological interpretation, and clinical use”, published 1996, Task Force of The European Society of Cardiology and The North American Society of Pacing and Electrophysiology, hereinafter referred to as “Task Force”, and elsewhere. Each R-wave (of the P-Q-R-S-T sequence in ECG terminology) represents a forceful contraction of the heart and creates the pulse. The time difference between beats is known as the R-R interval. The beat-to-beat variability is affected by stimulation from the autonomic nervous system on the heart. The autonomic nervous system has two branches that usually work in an antagonistic manner: the sympathetic nervous system (SNS) that stimulates internal organs in preparation for “fight or flight” behavior, and the parasympathetic nervous system (PNS), that is associated with relaxation and allows humans to “rest and digest”. The higher the level of PNS activity, the greater the freedom from physiological and psychological stress the person is experiencing. A good way to observe PNS activity non-invasively is to examine changes in heart rate in response to breathing. Heart rate increases during inhalation and decreases during expiration. This effect is known as respiratory sinus arrhythmia (RSA), and the changes in heart rate depend on breathing rate and depth (among other variables). High levels of PNS activity (as measured via RSA) indicate that the person is exhibiting a freedom from physiological and psychological stress. High parasympathetic HRV is known to be cardioprotective in the sense that persons with consistently high measurements of this parameter are less likely to suffer potentially fatal cardiac arrhythmia than persons with lower values. This principle may be used to measure relative changes in the effect of physical or mental stressors on an individual when their HRV is measured on a frequent (e.g., daily) basis, as described below in relation to one aspect of the invention.
The analysis and use of heart rate variability measurements has been established for more than 40 years, and has proven to have diagnostic and prognostic value in many clinical applications including, but not limited to: state of recovery following acute heart attack, work related stress level assessment, and fetal distress. Heart rate variability has also been used retrospectively to examine the impact of exercise and training that have already been performed by athletes, sedentary users and patients with existing cardiovascular disease.
At the current time, HRV is conventionally measured according to the Guidelines stated by the Task Force, by capturing ECG data gathered over periods ranging from 5 mins to 24 hrs using a clinical ECG apparatus or, for longer periods, a Holter recorder. The captured signals are first digitized and then processed using a frequency domain transformation in order to separate the markers of the two separate branches of the autonomic nervous system. In particular, the so called “High Frequency” band (defined as 0.15 Hz to 0.4 Hz) is associated with the parasympathetic (vagal) branch of the autonomic nervous system. The Task Force recommends short-term recordings of 5 min made under physiologically stable conditions processed by frequency-domain methods. A frequency domain transformation (e.g., Fast Fourier Transform) is recommended by the Task Force to have at least 512 data points in order to allow the accurate determination of power within the High Frequency band. This means in practice gathering approximately 6-9 mins of data for persons with resting pulse rates in the range 60-80 bpm. Interpolation of a lower number of data samples (e.g., 400) may also be used, but this still results in practical measurements of 5 minutes.
The present invention relies on the following principles:
1. Correlation between i) the current state of cardiovascular fitness and recovery in an individual, and ii) activity level of the parasympathetic (vagal) branch of the autonomic nervous system, observed conventionally via power level of HF modulation (HF power) in a frequency domain transformation of heart beat interval (R-R) data under consistent environmental and physiological conditions.
2. Mathematically proven, and empirically confirmed, relationship between time domain measured RMSSD and the more often used frequency domain measured HF power, recommended by the Task Force. RMSSD (ms), is the Root Mean Square Successive Difference, i.e., the square root of the mean of the sum of the squares of differences between adjacent normal RR intervals from short-term recordings or from an entire 24 hour electrocardiogram recording. This time-domain measure strongly reflects PNS modulation, and has also been shown to be mathematically equivalent to the Poincare SD1 measure times 1.414. The RMSSD measure has mainly been replaced in studies during the last 10 years by frequency domain analyses, since the latter give a more complete insight from the R-R interval data into the workings of multiple branches of the ANS. Nonetheless, the RMSSD time domain measure can give faster, accurate results when only the activity of the PNS is desired to be known.
3. High degree of correlation, and predictive accuracy, of ultra short term (e.g., 30 sec) versus more usual short term (5 min or greater) RMSSD measurements. Reference: “Accuracy of ultra short heart rate variability measurements”, Engineering in Medicine and Biology Society, 2003, Proceedings of the 25th Annual International Conference of the IEEE. For example, the Task Force recommends short term RMSSD measurements.
4. Use of low frequency (typically <0.15 Hz), controlled deep breathing techniques during the measurement period in order to reduce the known impact of respiration rate and depth on HRV measurements, and also to help the user to relax, focusing the measurement result on physical (rather than mental) stress changes.
5. When possible, use of a static, standing position for measurement taking, avoiding supine saturation effects that can otherwise reduce the range of observed HRV measurements, especially in athletic individuals.
6. Recommendation to the user to take the daily measurement at about the same time each day, to further reduce variability, in this case caused by time of day induced (circadian) HRV variation.